The present invention deals generally with the biasing of a balanced party line or bidirectional transceiver so that such will operate for communication over a moderate range of offset voltages as measured at each of devices communicating bidirectionally or communicating as distributed upon a party line, which offset voltages are commonly referred to as common-mode voltages. The present invention specifically deals with a resistor and zener biasing network which allows a standard tri-state driver and accompanying standard receiver, collectively forming a transceiver, to operate within a common-mode voltage range (nominally plus or minus 8 volts) which is a greater common-mode voltage than that differential signal voltage which the transceiver does elsewise communicate at, and which common-mode voltage is at a level which might elsewise but for the biasing network damage or destroy the driver.
Standard high speed unidirectional quad receiver and driver circuit packages, such as the Am26LS31 tri-state driver and the Am26LS32 differential receiver available from Advanced Micro Devices, are commonly combined to produce balanced bidirectional transceiver circuits. In order to reduce the number of interconnecting input/output cables, "party line" configurations are normally used wherein each balanced tri-state driver/receiver is placed on the same pair of I/O conductors, thereby forming a transceiver and removing the requirement for separate transmit and receive paths between the collective transceivers. Several transceivers associated with different devices and distributed along the party line communication path may then all communicate on the selfsame single party line interconnecting path. If a balanced party line transceiver is to function as a useful input/output device in real world system, it must be able to operate over a moderate range of offset voltages as are existent between the chassis grounds of the host communicating devices. Such offset voltages arise because a universal ground is imperfect in a dispersed system, and certain voltage potential differences are prone to arise between the grounds at the various distributed devices. These offset voltages are referred to as common-mode voltages since it appears to each transceiver that a d.c. voltage bias is impressed upon the differential data signal. Most new military interfaces, including balanced party line configuration interfaces, are required to operate over a standard plus or minus 8 volts d.c. common-mode voltage range.
Integrated circuit packages, particularly quad-receiver integrated circuit packages, are not currently available which will operate with the military standard required plus or minus 8 volts d.c. common-mode voltage range. This is due to the inability of tri-state drivers, such as the aforementioned Am26LS31 of Advanced Micro Devices, to remain in a high impedance state, allowing the transceiver to operate at a receiver, in the presence of a common-mode voltage bias. Furthermore to problems with reception, if a negative common-mode voltage is applied to a driver in a high impedance state, then it will clamp at one diode drop below ground. If the current draw, due to the common-mode voltage, is sufficiently large from the driver in this state then parasitic elements will cause such driver to enter a "latch-up" condition which may damage or destroy the driver. These breakdown and latch-up points typically occur for tri-state drivers within those operating regions dictated by the common-mode voltage requirements.